The present invention relates to polyepoxide compositions, and in particular, to those polyepoxide compositions which contain grafted siloxane polymers dispersed therein.
Epoxy resins have a spectrum of properties that are well adapted for use in coatings, adhesives, fiber reinforced laminates, composites, engineering plastics, and specialty applications, such as potting resins and mortars. Among those properties are corrosion and solvent resistance, good adhesion and electrical properties, good dimensional stability, hardness, low shrinkage on cure and many other beneficial properties.
Unfortunately, a major disadvantage of epoxy resins concerns an undesirable brittleness. A partial solution to such a problem has been the addition of reactive liquid polymers (RLP). These RLP's are generally elastomers, such as carboxyl-terminated butadiene-acrylonitrile copolymers, which precipitate out of solution during cure of the polyepoxide. The precipitation results in the formation of discrete elastomer particles or domains which toughen the resin. Although toughening the cured resin, such a technique results in a significantly lowered softening temperature. Particle size is a major factor which determines the mechanical properties. The selectivity and reactivity of the functional groups are critical in the formation of the particles. Also, the curing conditions seriously affect their size and structure. Another disadvantage is that the RLP is limited to low molecular weights for good mixing and low viscosity. Yet another disadvantage involves the relatively poor thermal and oxidative stability of polymers comprising polymerized monomers such as butadiene.
Curable epoxy resin compositions containing acrylate rubbers are disclosed in European Patent Application No. 78527. For example, the reference discloses polyepoxides containing rubbers prepared from butyl acrylate. Unfortunately, said rubbers are soluble in the polyepoxide continuous phase at temperatures above about 51.degree. C., and in some instances at room temperature. Thus, undesirable softening of the cured resin can readily occur. In addition, it is difficult to control particle size of the dispersed phase polymerizate because dissolving and reprecipitation of said polymerizate is difficult to control. Control of parameters such as particle size of the polymerizate are critical in optimizing mechanical properties of the composition.
Curable blends of epoxy resins and organopolysiloxanes are disclosed in U.S. Pat. Nos. 3,843,577 and 3,926,885. Such epoxy resin composites are disclosed as being dispersions of organopolysiloxanes in an epoxy resin continuous phase. Dispersing agents are employed in preparing such composites. Although the composites are disclosed as having self-lubricating properties, the mechanical properties of such composites are not as great as would be desirable.
It would be highly desirable to provide a process for significantly improving the toughness of epoxy resins without sacrificing the other properties which would extend the utility of said resins. It would be particularly desirable to provide a control over dispersed particle size, to provide dispersed particles which are highly incompatible with the continuous epoxy phase and to provide a product having high heat distortion temperatures in the cured form. In addition, it would be desirable to provide a more processable polymer-modified epoxy resin (i.e., lower viscosity than that exhibited by RLP modified resins).
Thus, in view of the deficiencies of the prior art, it would be highly desirable to provide a stable dispersion of a polymer in an epoxy compound, wherein the dispersed polymer phase does not interfere with the desirable properties of the epoxy compound; and which dispersion when cured exhibits (1) a desirably high glass transition temperature; and (2) a good toughness, as provided by a dispersed polymer phase.